Browsing by Author "Steyn, Willem J."

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    Red colour development and loss in pear fruit
    (Stellenbosch : Stellenbosch University, 2003-03) Steyn, Willem J.; Jacobs, G.; Wand, S. J. E.; Holcroft, D. M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Downgrading of fruit due to insufficient red colour has limited the profitability of lucrative blushed pear cultivars (Pyrus communis L.). In 'Rosemarie', poor fruit colour has been ascribed to pre-harvest red colour loss during periods of high temperature. The regulation of colour development in pears has not been studied and, in addition, little is known about anthocyanin degradation in attached fruit. Changes in colour were recorded and phenylalanine ammonia-lyase (PAL) and UDPGalactose: flavonoid-3-0-glycosyltransferase (UFGT) activities assessed in response to cold fronts and during fruit development in order to establish the regulation of colour development in red and blushed pear cultivars. Best red colour was generally attained a month or more before harvest whereafter red colour faded towards harvest. Unlike in some other fruits, UFGT activity apparently did not limit colour development whereas fading of red colour towards harvest might relate to decreasing PAL activity. 'Rosemarie' colour fluctuated considerably, increasing with cold fronts and decreasing during intermittent warmer periods, while red colour was more stable in other cultivars. PAL and UFGT activities in 'Rosemarie' increased in response to low temperatures, but were unaffected in 'Bon Rouge'. We concluded that anthocyanin synthesis in 'Rosemarie' requires low temperatures while colour development in 'Bon Rouge' and probably also other cultivars is primarily regulated by endogenous factors. Detached pome fruit were used to study temperature and light effects on anthocyanin degradation and fruit colour and to assess the modifying effect of anthocyanin concentration on colour loss. Anthocyanin degradation and red colour loss increased linearly between 10°C and 30°C. Irradiation further increased the rate of degradation and colour loss. The rate of colour loss depended on anthocyanin concentration, being much faster in fruit with high compared to fruit with low pigment levels. This was ascribed to the exponential relationship between anthocyanin concentration and hue at high pigment levels and the linear relationship at lower pigment levels. Anthocyanin degradation and pre-harvest red colour loss in 'Rosemarie' was quantitatively confirmed and corresponded with a warm period during fruit development. Based on these data, we attributed the susceptibility of 'Rosemarie' to pre-harvest colour loss to low anthocyanin concentrations in its peel that allow the visualisation of net anthocyanin degradation at high temperatures. Overhead evaporative cooling (EG) as measure to improve red colour in blushed pears was evaluated. 'Rosemarie' fruit that received pulsed EG applications from two weeks before harvest at air temperatures exceeding 28°G were redder than control fruit at harvest. EG had no effect on 'Forelle' colour. Though EG could be used to improve 'Rosemarie' fruit colour in warm production areas, its effect was relatively small compared to colour change in response to temperature. Lastly, we assessed the photoprotective function of anthocyanin in pear peel. Photoinhibition was evident in exposed faces of pears under natural conditions. The extent of photoinhibition increased with decreasing redness of peel and was maintained after photoinhibitory treatment. Although anthocyanin was apparently able to afford photoprotection at 40oG, we argued against this as a general function. There were indications that photoprotection was associated, but not necessarily due to light attenuation by anthocyanin.
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    Red colour development and loss in pears
    (International Society for Horticultural Science -- ISHS, 2005-04) Steyn, Willem J.; Wand, S. J. E.; Holcroft, D. M.; Jacobs, G.
    The endogenous and environmental regulation of red colour development in blushed and fully red pears is reviewed. Colour development in pears has an underlying developmental component. Generally, highest anthocyanin concentrations are attained in immature pears and colour tends to fade towards harvest. This is contrary to most other crop species where maximum pigmentation and colour are attained in ripe fruit and may relate to the photoprotective ability of anthocyanins. Because of this pigmentation pattern, net anthocyanin degradation at high temperatures results in pre-harvest red colour loss in susceptible pear cultivars. Susceptibility depends on the capacity to accumulate anthocyanin and on whether low temperatures are required for anthocyanin synthesis. Unlike apples, where red colour development in all cultivars seems to require or benefit from low temperatures, not all pear cultivars seem to respond to low temperatures. Light appears to have two opposing effects in pears, being required for anthocyanin synthesis, but also apparently increasing red colour loss through increased degradation of anthocyanin.
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    Use of evaporative cooling to improve ' Rosemarie' and ' Forelle' pear fruit blush colour and quality
    (International Society for Horticultural Science -- ISHS, 2005-04) Wand, S. J. E.; Steyn, Willem J.; Holcroft, D. M.; Mdluli, M. J.; Van Den Dool, K.; Jacobs, G.
    In Pyrus communis ‘Rosemarie’, a potentially lucrative blushed pear cultivar, poor fruit colour has been ascribed to pre-harvest red colour loss during periods of high temperature. High temperatures combined with high irradiances also lead to sunburn on pome fruit. The use of pulsed overhead evaporative cooling (EC) to improve red colour and reduce sunburn in two blushed pear cultivars was evaluated over four seasons (2000/01 to 2003/04) in Stellenbosch, South Africa. ‘Rosemarie’ fruit blush was usually but not always improved under EC, with a late application (starting two weeks before harvest) yielding the best improvement. Peel anthocyanin concentrations were increased under EC. Incidence of sunburn was reduced during some years, but increased in EC-treated fruit during 2003/04 due to system failure on a hot day. EC initiated early in fruit development initially led to larger fruit with a lower TSS concentration and firmness, but this effect was gradually eliminated following reductions in EC water use. EC had no effect on ‘Forelle’ pear fruit colour or mass, but reduced firmness and TSS when started early in the season. Sunburn was only recorded in EC-treated fruit during 2002/03 due to system failure on a warm day. Though EC could be used to improve ‘Rosemarie’ fruit colour in warm production areas, its effect was relatively small compared to colour change in response to fluctuating temperature. The increased risk of sunburn during system ‘downtime’ highlights the requirement for a highly reliable system.